How to get acoustic barriers just right
Noise pollution has been found to have harmful impacts on our physical health and wellbeing, with problems including poor sleep and moods swings, to more severe health problems including cardiovascular diseases. The impact of noise is often most prevalent in busy urban areas or industrial sites, and with the current housing crisis, more and more residential developments are popping up in less-than-ideal environments. One very common solution to this problem is noise barriers, or acoustic screens which can reduce the effect of noise from roads, rail and industrial sources on noise-sensitive receptors, such as residential properties or schools.
Internal noise level limits within buildings can be achieved with mitigation, through careful glazing and ventilation design. However, meeting desirable noise levels in external areas such as gardens or balconies might require acoustic screening.
What’s going on?
Sound propagates from a source, such as a car, as expanding pressure waves. Blocking the direct path from the source to the receptor can significantly reduce the noise impact at the receiver. However, it is not quite that simple, the pressure waves hitting the top of the barrier will be diffracted and some of this sound will still be received at the receptor. A small amount of the incident sound could pass through the barrier and still impact the receiver. This is why the acoustic barrier needs to be sufficiently dense and continuous to reduce this transmitter sound to an insignificant level compared with the sound diffracted at the top of the barrier.
An acoustic barrier requires a minimum mass of at least 10kg/m2 to prevent sound travelling straight through it (for reference a domestic timber fence is in the region of 4kg/m2). Any mass above 15kg/m2 is unlikely to provide a significant sound reduction improvement, as the performance will be limited by sound diffracting over the top of the barrier.
A key acoustic consideration when designing an acoustic barrier is the height and length required for the project. Simply put, the taller and wider the barrier, the greater sound reduction that can be achieved. Like with the mass reaching optimum performance at around 15kg/m2, the barrier length must extend far beyond the receptor to avoid its performance being compromised by noise diffracting around the ends of the barrier.
To achieve the necessary performance from an acoustic barrier it will need to be completely imperforate so that sound cannot travel through it. Hit and miss fencing, slatted fencing or foliage are not acoustic barriers as sound passes through the gaps, even if they are visual screens.
Along with the height and length, barrier location is a key acoustic consideration. Optimum performance can be achieved by maximising the difference of the direct line between the source and the receiver and the diffracted path over the top of the barrier. On level ground usually it is best, acoustically speaking, to place the barrier as close as possible to the noise source. This cannot always be achieved as roads and railways could be in cuttings, for this scenario placing the barrier further away from the source at the top of the slope might be the better solution.
Acoustic considerations unfortunately are not the only element at play when using acoustic barriers or screens. What may end up being the perfect location for a barrier may also impact upon visual aspects. As barrier sizes increase it is more and more important that the screen is designed in collaboration with the planners and landscape architects, so that materials, colour and shape can all be considered early doors. With early consideration at the design stage, greening solutions can be explored such as climbing plants. This can mean that acoustic requirements are achieved, and visual impact can be reduced.
It should be noted that, each application is different and site restrictions will dictate the overall result. The important takeaway is that ideally at the earliest stages of planning, any requirement for acoustic screening is established and included within the masterplan from the outset.
SLR takes a balanced approach to specifying mitigation measures, taking into account a broad view of the project, with considerations of not just noise but other factors too, such as visual. At SLR we can provide advice to achieve the best scheme for site, no matter how challenging and problematic.